Acinetobacter baumannii is a major nosocomial and combat related pathogen primarily associated with respiratory and wound infections. Ubiquitous in the environment, this pathogen is highly resistant to desiccation and antibiotics leading many healthcare institutions to adopt strict screening protocols of patients admitted to the hospital to control spread. Despite extensive literature demonstrating its ability to colonize the human GI tract, and at least one study proposing a link between GI tract colonization and acquisition of antibiotic resistance, no empirical studies have been reported to study the mechanism of GI tract colonization. Recently, our laboratory set out to address this shortcoming utilizing a murine oral GI challenge model. To that end, we observed enhanced GI tract colonization and infection in the presence of SIgA, contrary to previous reports suggesting the bacteria proteolytically degraded SIgA to neutralize its protective barrier function. We observed reduction of the disulfide bonds and release of secretory component (SC) from the immunoglobulin. Absence of SIgA resulted in both decreased bacterial attachment and mortality in IgA deficient animals. In vitro assays showed that gene expression of the bacterial enzyme thioredoxin-A (TrxA) was up regulated in response to SIgA exposure. Consequently, inhibition of this enzyme with an asymmetric disulfide compound (PX-12), reported to specifically inhibit thioredoxin and thioredoxin-fold motif (-C-X-X-C-) containing proteins, resulted in decreased bacterial attachment to intestinal epithelial sections obtained from WT mice. Furthermore, deletion of the trxA gene from a MDR A. baumannii clinical isolate (?trxA) significantly reduced virulence. Based on these observations, we hypothesize that reduction of SIgA by A. baumannii secreted TrxA enhances bacterial colonization, thus immuno- and drug-targeting TrxA provides a novel approach to control of this important MDR pathogen. The proposed study will test this central hypothesis by first delineation of this novel Acinetobacter pathogenic mechanism of TrxA-mediated dissociation of SC from SIgA and enhancement of bacterial attachment, using comparisons of WT MDR clinical isolate with our ?trxA mutant. The unique pathogenic mechanism will be further assessed/confirmed with inhibition of TrxA function by antibody and small molecule drug treatment. The results of the proposed antibody and drug studies will provide a foundation for the development of therapeutic treatments (e.g. monoclonal antibodies, refined small molecule asymmetric compounds with improved potency) for this important MDR pathogen.